1. Field of the Invention
The present invention relates to a driving circuit for ink jet printing head using piezoelectric actuator to drive an ink jet printing head and more particularly to a driving circuit for ink jet printing head which modulates the diameter of ink droplets ejected from nozzles (droplet-diameter modulation) based on gradation-represented printing data, thereby changing the size of dots formed on printing paper in order to improve the gradation of characters and images.
2. Description of the Related Art
An example of an ink jet head driving circuit which improves by droplet-diameter modulation the gradation of characters and images by changing the size of dots formed on recording paper is disclosed for example in Japanese Laid-Open Patent Application No. Hei9-11457. This ink jet head driving circuit is provided with common waveform generating means which generates four kinds of driving waveform signals S3 through S0 (see (a)-(d) of FIG. 15) which correspond to a total of four cases consisting of three cases where three sizes of dots are formed and one case where no ink is ejected.
One example of this common waveform generating means is disclosed in Japanese Laid-Open patent Application No. Hei2-16544 (Japanese Patent Gazette No. 2689548), the electric configuration of which is shown in FIG. 16. The common waveform generating means is composed of a waveform generating unit 1 and a current amplifier unit 2.
The waveform generating unit 1 roughly is composed of constant current sources 3 and 4 and a capacitor 5. The constant current source is composed of transistors 6 and 7, a resistor 8, and a constant voltage diode 9, while the constant current source is composed of transistors 10 and 11, a resistor 12, and a constant voltage diode 13. When a H-level control signal SA is supplied to the waveform generating unit 1, an electric current flowing from the transistor 6 to the capacitor 5 is forcedly cut off; if another H-level control signal SB is supplied to it, the constant current source 3 charges the capacitor 5; and if another H-level control signal SC is supplied to it, the constant current source 4 discharges the capacitor 5, thereby generating four kinds of driving waveform signals S3 through S0 shown in (a)-(d) of FIG. 15 respectively. The current amplifier unit 2, which is of a single ended push-pull (SEPP) type, roughly is composed of an NPN-type transistor 14 and a PNP-type transistor 15 which are connected in a emitter-follower configuration, with which voltage corresponding to the above-mentioned driving waveform signals S3 through S0 is applied to a plurality of piezoelectric actuators (not shown) connected in parallel at an output terminal 16 without being influenced by the number of these actuators so that these actuators may be charged and discharged.
Thus, as disclosed in the above-mentioned Japanese Laid-Open Patent Application No. Hei9-11457 describes, it is possible to generate the driving waveform signals S3 through S0 shown in FIG. 15 by using the circuit (see FIG. 16) disclosed as one example of the common waveform generating means disclosed in Japanese Patent Gazette No. 2689548. In the waveform generating unit 1 shown in FIG. 16, however, a current which charges the piezoelectric actuator is determined by the resistor 8 and the constant voltage diode 9 which make up the constant current source 3 and a current which discharges the piezoelectric actuator is determined by the resistor 12 and the constant voltage diode 13, so that in order to generate four kinds of driving waveform signals S3 through S0 shown in FIG. 15, it is actually necessary to appropriately switch the values of the resistors 8 and 12 or change the collector voltage of the transistors 7 and 11. This presents a disadvantage of more complicated circuits concerned.
Also, the above-mentioned conventional ink jet head driving circuit, which charges and discharges the capacitor 5 shown in FIG. 16 to generate the driving waveform signals S3 through S0, has high voltage of several tens of volt applied to the capacitor 5 and also needs to be provided with a charging path and a discharging path separately, thus presenting a disadvantage of requiring a number of separate elements which cannot be integrated. Moreover, That driving circuit has a disadvantage of restricted selection of elements because it requires elements with good frequency response to generate driving waveform having a high voltage slew-rate (dV/dt) value.
Also, a preferable mode is one wherein capacitance of 3000 pF each, so that when for example 300 piezoelectric actuators are driven at the same time, the total capacitance amounts to as large as 0.9 xcexcF. With this, if a simple SEPP type of current amplifier is configured such as shown in FIG. 16, the capacitive load is as large as 0.9 xcexcF, so that when, moreover, a driving waveform signal with a high voltage slew-rate (dV/dt) is applied, the current amplifier unit 2 may oscillate at around several MHz. In the event of such oscillation, the transistors are excessively heated and may be destroyed, thus presenting another problem.
Also, in the current amplifier unit 2 shown in FIG. 16, even when no printing is performed, that is, when the transistor 15 is in the OFF state, a slight leakage current flows between the collector and the emitter of the transistor 15, so that it is difficult to hold at a constant value the voltage applied to the piezoelectric actuators. Therefore, when the DC voltage is gradually decreased, as shown by a dash-and-dot line in FIG. 7, which is applied to the piezoelectric actuators when ink is ejected from the second time onward, a displacement of the piezoelectric actuators, which is proportional to the voltage, is also decreased, thus disabling the ejection of ink, which presents another problem.
If the DC voltage applied to the piezoelectric actuators is increased gradually, on the other hand, ink may be ejected undesirably, which presents another problem.
In view of the above, it is an object of the present invention to provide a driving circuit for ink jet printing head that can be easily configured even with inexpensive elements, that does not malfunction, and that can generate desired driving waveform signals to drive piezoelectric actuators with a large capacitive load.
According to an aspect of the present invention, there is provided a driving circuit for ink jet printing head which has at least one nozzle and at least one pressure producing chamber and which, when printing, applies a driving waveform signal to at least one piezoelectric actuator provided at position corresponding to the pressure producing chamber to rapidly change a volume of the pressure producing chamber filled with ink, thereby ejecting ink droplets from the nozzle, further including:
storage means for storing driving waveform information about driving waveform signals for each diameter of the ink droplets; a plurality of waveform control means which is provided for each diameter of the ink droplets and which reads out the driving waveform information according to a waveform of corresponding driving waveform signals and then sequentially output the driving waveform information;
a plurality of waveform generating means which is provided for each diameter of the ink droplets, for generating a corresponding driving waveform signal by converting driving waveform information provided sequentially from the waveform control means into analog information and then conducting integration operation on the analog information; and
driving means which selects one driving waveform signal of a plurality of driving waveform signals output from the plurality of waveform generating means and applies the one driving waveform signal to the piezoelectric actuator.
In the foregoing, a preferable mode is one wherein the driving waveform information has time information about time of change point of corresponding driving waveform signals and voltage information about voltage of the change point or current information which is a differential value of the voltage information in terms of time; and
each waveform control means sequentially outputs the voltage information or the current information according to the time information.
Also, a preferable mode is one wherein each waveform generating means has a digital/analog converter which converts the voltage information or the current information into an analog signal, an integrator which has an operational amplifier and an integrating capacitor to perform integration operations on the analog signal, a negative feed-back unit which gives a negative feed-back to the operational amplifier so as to hold an output voltage of the waveform generating means to a zero potential before stating of and after termination of printing and to a prescribed bias potential which provides a reference of contraction and expansion of the piezoelectric actuator at time point of not printing during printing operations, and a negative feed-back cut-off unit which cuts off the negative feed-back to ground a positive input terminal of the operational amplifier.
Also, a preferable mode is one that wherein further having a plurality of power amplification means which is provided for each diameter of the ink droplets, for power-amplifying driving waveform signals output from corresponding waveform generating means and supplying the signal to the driving means, wherein each power amplification means has a differential amplification means which differential-amplifies corresponding driving waveform signals, a voltage amplification unit which voltage-amplifies an output signal of the differential amplification unit, a single-ended push-pull type current amplification unit which current-amplifies an output signal of the voltage amplification unit, and a negative feedback unit which gives a negative feed-back to the differential amplification unit from the current amplification unit.
Also, a preferable mode is one wherein the driving means has a data transmission unit, a data receiving unit, and a plurality of transfer gates provided for each diameter of the ink droplets for each piezoelectric actuator;
the data transmission unit sends at least gradation information of printing data to the data receiving unit; and
the data receiving unit is provided together with the plurality of transfer gates near the piezoelectric actuators, to turn corresponding transfer gates ON or OFF based on gradation information sent from the data transmission unit.
Also, a preferable mode is one wherein at least the plurality of waveform control means and the data transmission unit are integrated into one unit.
Furthermore, a preferable mode is one wherein a temperature sensor is provided near the piezoelectric actuator;
the storage means stores driving waveform information for each diameter of the ink droplets for each temperature of the piezoelectric actuator; and
each waveform control means reads out the driving waveform information from the storage means based on a temperature signal sent from the temperature sensor.
With the above construction, it is possible to configure circuits easily and with inexpensive elements and also to generate desired driving waveform signals which drive piezoelectric actuators with a large capacitive load.
Also, it is possible to eject ink droplets in a stable manner irrespective of changes in the viscosity of ink due to changes in the temperature of the ink jet printing heads.